The general procedure of forming a wellbore, such as an oil and gas wellbore, includes drilling a bore into the earth until the surrounding formation requires support or where the characteristics of the formation requires it to be sealed-off from the drilled bore. At this stage a length or string of tubing, conventionally called casing, is run into the formed bore and is cemented in place. The casing and cement are therefore utilized to support and seal the bore. If the bore is to be advanced then a smaller diameter drilling assembly is run through the cemented casing and the procedure is repeated until the required or total depth is reached. The resulting cased wellbore therefore has a stepped profile with the final casing string having a significantly reduced diameter.
Conventionally, a cement or casing shoe is mounted on the lower end of each casing string, wherein the shoe defines a rounded end which assists smooth running of the casing into the bore, and also incorporates flow passages which permit cement delivered through the casing to exit and flow into the annulus formed between the casing and the bore wall. A float collar is positioned one or two joints above the bottom of the casing string and contains a check valve to permit fluid to pass downward but not upward through the casing.
When the casing string is fully run into the drilled bore, a calculated volume of cement is displaced downwardly through the casing, through the cement shoe and into the annulus. Typically, the volume of cement is contained between lower and upper cement plugs and is displaced by a fluid, such as drilling mud, pumped from surface. The lower plug prevents contamination of the cement from fluids ahead of the cement, and the upper plug prevents contamination of the cement from the displacing fluid. During a cementing operation, the lower cement plug will eventually land on top of the float collar and increasing pressure will burst a disk in the lower plug opening a passage therethrough, permitting the top plug to displace the cement through the lower plug, float collar and cement shoe and into the annulus.
Following a cementing operation the internals of the float collar and cement shoe must be drilled through to advance the depth of the bore. However, such drilling imparts vibration and other mechanical forces into the cement surrounding the cement shoe which may therefore be adversely affected.
In certain cases, such as in offshore deepwater areas, for example in the Gulf of Mexico, the formation is particularly weak and requires many casing strings to be run in to ensure sufficient bore support and sealing. However, if many casing strings are required then the reduction in bore diameter will be significant, which is undesirable. In order to minimize significant bore size reductions attempts are made to utilize casing strings which are as close to the drilled bore diameter as possible, resulting in a reduced annulus area between the casing and the bore. However, a reduced annulus area creates higher circulating pressures that can break down the formation in view of the low formation fracture pressure and formation strength, which creates losses and adverse well control situations. It is known in the art to address this problem by underreaming the bore to make it larger to thus lower the circulation pressure. However, this action makes it more difficult to create a good and strong cement job, particularly around the casing shoe.
Further concerns in the oil and gas exploration and production industry relate to the delivery of materials, such as chemicals and the like, to specific regions of a wellbore. In many cases it is preferred that the material to be delivered is isolated from wellbore fluids, at least until the required location in the wellbore is reached. This may be achieved in the art by utilizing coiled tubing, for example, which is run into the wellbore to create an uncontaminated flow-path. However, running in coiled tubing and subsequently delivering fluid through the tubing is time consuming and requires a significant amount of dedicated space on a rig platform, which is at a premium.